Process for the MIG welding of aluminum and its alloys with a shielding gas of the Ar/He/O2 type

ABSTRACT

The invention relates to a process for the MIG welding, in spray mode without current modulation or in pulsed mode, of aluminum and aluminum alloys, with the use of a gas shield for at least part of the welding zone. According to the process of the invention, the gas shield is a gas mixture consisting, by volume, of from 0.8% to 1.80% oxygen and from 15% to 99.99% helium, and possibly containing a balance of argon. Preferably, the gas mixture contains at least 1% oxygen.

[0001] The present invention relates to an MIG (Metal Inert Gas) processfor the electric-arc welding of aluminum and aluminum alloys, in pulsedmode or spray mode, that is to say with axial spraying, with theexception of a process in spray mode with modulated current.

[0002] The MIG electric-arc welding process is widely used in industry,including that of welding aluminum.

[0003] Shielding gases play an essential role in the performance of thisprocess.

[0004] Thus, argon does not allow high welding speeds and may give rise,in automatic welding, to arc instabilities above 500 A. Thecharacteristic shape of the narrow penetrations obtained with argon inspray mode is ill-suited to welding by interpenetration.

[0005] Moreover, argon/helium (hereafter Ar/He) mixtures and helium byitself make it possible to increase the depth of penetration and itsroot width, and hence make it possible to dispense with expensivepreparations, the more so the higher the helium content in the Ar/Hemixtures.

[0006] In other words, for a constant thickness, helium therefore allowswelding speeds which increase with its content.

[0007] However, although the compactness of the beads is also generallyimproved by the presence of helium, this is to the detriment of theappearance of the beads, which are less shiny than with pure argon.

[0008] Ar/He mixtures therefore present an appreciable advantage interms of quality and productivity, both in manual welding (for examplefor a mixture of the Ar/20% He type) and in automatic welding (forexample for mixtures of the Ar/50% to 70% He type) with, however, a notinsignificant cost associated with the helium content.

[0009] For applications not necessarily having to meet these twocriteria, it may be judicious to consider other types of mixtures.

[0010] Thus, document EP-A-639 423 proposes, for TIG and MIG processes,to use a welding gas of the argon or argon/helium type containing,furthermore, from 100 to 1000 ppm by volume of CO₂ and/or O₂.

[0011] Furthermore, document DE-A-4241982 proposes to use argon or anAr/He mixture to which has also been added from 80 to 250 ppm by volumeof nitrogen.

[0012] Document EP-A-442 475 recommends welding with a consumableelectrode using a gas mixture consisting of 0.5 to 1.25 vol % carbondioxide, 30 to 40 vol % helium and the balance being argon.

[0013] Document U.S. Pat. No. 4,071,732 relates to a mixture formed froman inert gas containing less than 30% carbon dioxide or less than 5%oxygen; however, the examples in this document describe only mixtures ofargon and carbon dioxide with contents of about 5 or 15%.

[0014] It should be emphasized that in terms of increasing theperformance, none of these processes is fully satisfactory from theindustrial standpoint.

[0015] Furthermore, in modulated spray MIG welding, that is to say withthe welding current being modulated, it has already been recommended touse a shielding gas or gas mixture formed from argon, helium or mixturesthereof, to which from 0.01% to 1.80% of carbon dioxide and/or oxygenhave been added, as described in EP-A-909 604. However, in this case,current modulations at a frequency of less than 60 Hz are applied to thecurrent so as to be able to degas the weld puddle in order to removegaseous inclusions therefrom, particularly diffusable hydrogen, liableto be found therein. This is because MIG processes in spray mode withcurrent modulations are used when it is desired to obtain a high qualityof the welded joint, but without actually needing to achieve a highwelding speed.

[0016] Consequently, the problem that arises is to improve the known MIGwelding processes not using modulation of the welding current,particularly MIG processes in unmodulated spray mode, that is to saywithout modulation of the welding current, and processes in pulsed mode,so as to be able to obtain high performance levels in terms ofproductivity and welding speed.

[0017] This is because MIG processes in unmodulated spray mode (withoutmodulation of the current) and those in pulsed mode are much moresuitable when it is desired to improve productivity rather than quality,that is to say the appearance of the welds.

[0018] Hitherto, MIG processes in unmodulated mode or in pulsed mode areused little or not at all for welding aluminum or its alloys when thegas shield contains oxygen.

[0019] This is because it is usually recognized that the presence ofoxygen in the gas shield may have a deleterious effect on the weld giventhat, when oxygen is incorporated into the gas shield, the oxygen caneasily combine with aluminum atoms and result in solid inclusions ofalumina (Al₂O₃) in the weld, which have a negative effect on themechanical properties of said weld. Moreover, this has been confirmedfor high oxygen contents, that is to say oxygen contents greater than2%, and for high carbon dioxide contents, that is to say, again,contents greater than 2%.

[0020] However, conversely, the presence of oxygen in the shielding gasstream results in productivity performance levels which are acceptablefrom the industrial standpoint.

[0021] It therefore follows that the problem that arises is to providean MIG welding process for aluminum and its alloys resulting both in ahigh and industrially acceptable productivity and a low level of aluminainclusions in the weld without a major or appreciable impact on themechanical properties of the welded joints.

[0022] The solution provided by the present invention therefore relieson a process for the MIG welding, in spray mode without currentmodulation or in pulsed mode, of aluminum and aluminum alloys, with theuse of a gas shield for at least part of the welding zone, wherein thegas shield is a gas mixture consisting of from 0.01% to 1.80% oxygen andfrom 20% to 98.2% helium, the balance possibly consisting of argon.

[0023] Further characteristics of the process of the invention are givenbelow:

[0024] the shielding gas contains from 0.9% to 1.80% oxygen and from 15%to 98.20% helium, the balance being argon;

[0025] the shielding gas mixture contains at least 1% oxygen, preferablyat least 1.1% oxygen, more preferably at least 1.2% oxygen;

[0026] the shielding gas mixture contains at most 1.70% oxygen,preferably at most 1.65% oxygen;

[0027] a solid meltable wire is used;

[0028] the welding speed is from 0.25 m/min. to 1.20 m/min., preferablyfrom 0.60 to 1 m/min.;

[0029] the wire speed is from 2.5 m/min. to 20 m/min., preferably from 4m/min. to 17 m/min.;

[0030] the mean welding current is from 40 A to 450 A and/or the meanwelding voltage is from 15 V to 40 V;

[0031] the process is in pulsed mode and/or the welding current is from120 A to 350 A and/or the mean welding voltage is from 20 V to 30 V;

[0032] the process is in spray mode and/or the welding current is from180 A to 450 A and/or the mean welding voltage is from 20 V to 39 V.

[0033] The present invention therefore relies on precise control of theoxygen content in the helium or an argon/helium mixture, it beingnecessary for the maximum oxygen content not to exceed about 1.80%, thegas mixture thus formed constituting the gas shield used whenimplementing the MIG process.

[0034] It should be emphasized that any MIG process in spray mode withmodulation of the welding current is excluded from the invention.

[0035] The invention will now be more clearly explained by means of thefollowing examples, given by way of illustration but implying nolimitation, the results of which are shown schematically in the figuresappended hereto.

EXAMPLES

[0036] In order to show the effectiveness of the MIG process accordingto the invention, several comparative trials were carried out.

[0037] Within the context of these trials, aluminum workpieces (5000 and6000 grades according to the NFEN 485, 487, 515 and 573 standards) werewelded using an MIG process in unmodulated spray mode and in pulsedmode, using a gas shield consisting of argon to which from 1 to 1.5% O₂was added.

[0038] The current generator was a 480 TR16 generator sold by La SoudureAutogene Francaise.

[0039] The meltable wire used as filler metal was, in all cases, a wire1.2 mm in diameter of the 5356 type (according to the AWS A5.10 or NFA50.403 standards).

[0040] Prior to welding, the aluminum workpieces were prepared bymechanical gouging.

[0041] The other welding parameters are given in Table I below, inwhich:

[0042] V_(wire) represents the feed speed of the meltable wire;

[0043] I_(p) represents the intensity of the peak current;

[0044] I_(b) represents the base current;

[0045] I_(mean) represents the mean current;

[0046] U_(p) represents the peak voltage;

[0047] U_(mean) represents the mean voltage;

[0048] F_(pulse) represents the current pulse frequency (in pulsed MIGmode);

[0049] T_(pulse) represents the current pulse time (in pulsed MIG mode).TABLE I Welding parameters for the two transfer modes Mode Spray PulsedGrade of the 5000 5000 aluminum workpiece Thickness (mm) 6 6 V_(wire)(m/min.) 12.5 9 I_(p) (A) — 330 I_(b) (A) — 110 I_(mean) (A) 220-240154-162 U_(p) (V) — 28 U_(mean) (V) 21-23 20-23 F_(pulse) (Hz) — 155T_(pulse) (ms) — 1.6

[0050] The results obtained are given below, after evaluating theperformance in terms of productivity (welding speed) and joint quality(compactness and appearance of the bead) and mechanical properties.

[0051] In Table II below, the increases in speed given for the 5000grade were determined with respect to the welding speeds for variousargon/helium mixtures and with or without the addition of O₂ and, by wayof comparison, the results obtained with argon to which O₂ was added,have also been given. These results are shown diagrammatically in FIG.1.

[0052] It may be seen that for identical O₂ additions, the increase inperformance, in terms of welding speed and in terms of penetration,remains equivalent for an Ar/He mixture and for argon alone. TABLE IIIncrease in performance in terms of welding speed for 5000 grade withrespect to an Ar/He mixture without the addition of O₂. Mode SprayPulsed Gas Ar Ar/20% He Ar/50% He Ar/70% He Ar Ar/20% He Ar/50% HeAr/70% He  0% O₂ 0.56 0.7 0.65 0.7 0.35 0.4 0.44 0.45 +1% O₂ 0.7 0.88n.d. n.d. 0.42 0.49 n.d. n.d. 25%  25.7% n.d. n.d. 20%  22.5% n.d. n.d.+1.5% O₂ 0.78 0.93 0.86 0.93 0.45 0.52 0.54 0.55 39.2% 32.8% 32.3% 32.8%28.5% 30%  22.7% 22.2%

[0053] Table II shows an increase in the welding speed for a weld withperfect penetration for a thickness of 6 mm.

[0054] Moreover, in order to evaluate the performance in terms of jointquality (compactness and appearance of the bead), X-ray examinationswere carried out on the 5000-grade workpieces, which did not reveal anyindications other than those normally encountered in MIG with argon orAr/He mixtures.

[0055] Complementarily, macrographic samples made it possible to reveal,by simple polishing, areas of reduced-size (about 0.01 mm) inclusions ofdifferent distribution and orientations.

[0056] These areas were identified in a scanning electron microscope asareas of alumina (Al₂O₃). Furthermore, the appearance of the beadsobtained with Ar/He/O₂ mixtures was substantially different from thoseproduced with an Ar/He mixture: the beads have a surface deposit whichis blackish depending on the retained O₂ content, which is removed bybrushing or with the aid of a rag.

[0057] Next, the mechanical properties were determined from leveledtransverse tensile and transverse bending test pieces. The tensileresults are shown in FIG. 2 in which the x-axis represents the O₂contents in the three Ar/He mixtures of different compositions and they-axis represents the tensile strength (Rm) values (in MPa) of the5000-grade assemblies.

[0058] The corresponding intrinsic values and the joint coefficients(strength of the melted metal/strength of the basis metal) are indicatedin Table III.

[0059] It will be noted that there is no significant degradation in themechanical properties of the melted zone depending on the increase inthe O₂ content. The maximum recorded reduction of 7 N/mm² (i.e. -2.6%)in spray mode with an Ar/He mixture remains less than that recorded forthe same transfer mode with Ar/O₂, namely 15 N/mm² (i.e. -5.5%). TABLEIII Mechanical properties on 5086-series assembles for Ar/He mixtureswith addition of O₂ Mode Spray Pulsed Gas Ar/20% He Ar/50% He Ar/70% HeAr/20% He Ar/50% He Ar/70% He  0% O₂ R_(m) (MPa) 271.12 263.2 266.47265.03 266 264.03 Joint coeff. 0.86 0.83 0.84 0.84 0.84 0.83 +1% O₂R_(m) (MPa) 269.92 n.d. n.d. 267.05 n.d. n.d. Joint coeff. 0.85 n.d.n.d. 0.84 n.d. n.d. +1.5% O₂ R_(m) (MPa) 264.72 260.12 259.71 266.61266.4 261.68 Joint coeff. 0.84 0.82 0.82 0.84 0.84 0.83 BM actual/315/275 guaranteed R_(m) (MPa)

[0060] All the tensile test pieces (having a thickness of 6 mm)systematically fractured in the melted metal, which is normal behaviorfor the 5000 series. Areas of fine dispersed black inclusions wererevealed on their fracture surfaces. These observations confirm theresults of the above macrographic examinations. They may become largerin size and greater in density according to the O₂ content, but, asmentioned above, their influence is not significant either on themechanical properties of the assemblies or on the deformability, sincethe results of the bending trials were satisfactory (complete absence ofeffects after bending through 180°).

[0061] It therefore follows (FIG. 1) that, for the same O₂ addition, again in performance (increase in the penetration or welding speed)remains comparable on an Ar/He mixture as on argon alone.

[0062] The addition of O₂ in a controlled amount (less than 2%) alsomakes it possible to leave out of account any particular preparation,such as grooving and separation, of workpieces up to 6 mm in thicknessto be welded together.

[0063] To these gains in productivity must also be added two othersubstantial operating advantages, namely the ease of ignition and thestability of the arc in the steady state. A comparative example showndiagrammatically in FIG. 3 demonstrates, in recordings, the substantialimprovement obtained in the stability of the current in spray mode withan Ar/He/O₂ mixture over spray mode with Ar/He (20 to 50% helium contentin both cases), that is to say without the addition of O₂, as shown bythe comparative examples in FIGS. 3 and 4 demonstrating, in Labviewrecordings, the improvements in the stability of the current in spraymode for an Ar/He mixture to which oxygen has been added.

[0064] These advantages have been demonstrated not only on pure aluminumbut also on 5000- and 6000-series aluminum alloys.

1. A process for the MIG welding, in spray mode without currentmodulation or in pulsed mode, of aluminum and aluminum alloys, with theuse of a gas shield for at least part of the welding zone, wherein thegas shield is a gas mixture consisting, by volume, of from 0.8% to 1.80%oxygen and from 15% to 98.20% helium, the balance possibly consisting ofargon.
 2. The process as claimed in claim 1, wherein the shielding gascontains from 0.9% to 1.80% oxygen and from 15% to 98.20% helium, thebalance being argon.
 3. The process as claimed in claim 1 or 2, whereinthe shielding gas mixture contains at least 1% oxygen, preferably atleast 1.1% oxygen, more preferably at least 1.2% oxygen.
 4. The processas claimed in claims 1 to 3, wherein the shielding gas mixture containsat most 1.70% oxygen, preferably at most 1.65% oxygen.
 5. The process asclaimed in claims 1 to 4, wherein a solid meltable wire is used.
 6. Theprocess as claimed in claims 1 to 5, wherein the welding speed is from0.25 m/min. to 2 m/min., preferably from 0.60 to 1.5 m/min.
 7. Theprocess as claimed in claims 1 to 6, wherein the wire speed is from 2.5m/min. to 25 m/min., preferably from 4 m/min. to 20 m/min.
 8. Theprocess as claimed in claims 1 to 7, wherein the mean welding current isfrom 40 A to 450 A and/or the mean welding voltage is from 15 V to 40 V.9. The process as claimed in claims 1 to 8, which is in pulsed modeand/or the welding current is from 120 A to 350 A and/or the meanwelding voltage is from 19 V to 32 V.
 10. The process as claimed inclaims 1 to 8, which is in spray mode and/or the welding current is from180 A to 450 A and/or the mean welding voltage is from 19 V to 39 V.